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Pacific Extratropical Precursors and Dynamic Mechanism of Two Types of Summer ENSO: Continuing and Emerging ENSO
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DOI：10.1175/JCLI-D-25-0049.1.png)
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Abstract
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El Ni & ntilde;o-Southern Oscillation (ENSO) exhibits great diversity in temporal evolution, which can be classified into two types of summer ENSO: continuing ENSO, persisting from the previous winter, and emerging ENSO, newly developing from late spring. However, the initial signal sources and dynamic processes underlying these two types of summer ENSO remain unclear. This study identifies the joint effects of extratropical North Pacific index (NPI) and extra-tropical South Pacific index (SPI) during the boreal winter as key initial signals affecting the diversity of ENSO evolution by modulating equatorial zonal wind anomalies. When boreal winter sea surface temperature anomalies (SSTAs) show a positive phase, a positive NPI + SPI in the preceding winter triggers equatorial easterly wind anomalies in spring. These anomalies reinforce ocean dynamic processes through positive feedback with local precipitation, leading to a strong SSTA tendency that facilitates the formation of emerging La Ni & ntilde;a. The converse initial conditions yield emerging El Ni & ntilde;o, driven primarily by the same dynamic processes. For continuing ENSO, a negative NPI + SPI following positive winter SSTAs induces equatorial westerly wind anomalies in spring, driving positive SSTAs and precipitation anomalies through wind-evaporation-SST feedback. This circulation configuration regulates SST dissipation through changes in latent heat and shortwave radiation flux anomalies. The air-sea interaction dominated by thermodynamic processes results in a weaker SSTA tendency, enabling the ENSO signal to persist into the following summer under the condition of greater ocean heat content in the preceding winter, exhibiting characteristics of continuing El Ni & ntilde;o. The converse initial conditions lead to continuing La Ni & ntilde;a through dominant thermodynamic processes. Our results provide a comprehensive and physically grounded understanding of the causal relationships behind the diversity of ENSO evolution.
Keywords:
Pacific Ocean
Atmosphere-ocean interaction
Atmospheric circulation
Dynamics
ENSO
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